When Daffy uses a balloon full of helium to try to outwit the dog, he is holding the balloon (which is tied around his waist) in his hand, and is firmly planted on the ground. When Daffy releases the balloon, he begins to float up and over the wall. If the balloon were able to lift him after he let it go, it should have been lifting him before he let it go.
Daffy uses a ladder, a pole vault, and a balloon to get over the mansion wall. But Daffy is a duck. He should be able to just fly over the wall.